From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CA120C83006 for ; Wed, 29 Apr 2020 15:23:33 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id A2FCC2063A for ; Wed, 29 Apr 2020 15:23:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A2FCC2063A Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jToYM-00051v-1n; Wed, 29 Apr 2020 15:23:14 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jToYK-00051q-Es for xen-devel@lists.xenproject.org; Wed, 29 Apr 2020 15:23:12 +0000 X-Inumbo-ID: 54d4b186-8a2d-11ea-995b-12813bfff9fa Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 54d4b186-8a2d-11ea-995b-12813bfff9fa; Wed, 29 Apr 2020 15:23:09 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 2BFF4AC7F; Wed, 29 Apr 2020 15:23:07 +0000 (UTC) Subject: Re: [PATCH] pvcalls: Document explicitly the padding for all arches To: Julien Grall References: <20200419104948.31200-1-julien@xen.org> <78288649-5930-9d01-bb8f-85e15406e4ef@xen.org> <6fc59120-664e-6a07-5196-57e1dbfb0dde@suse.com> <240bc5e8-f8fd-217a-fa10-7628ac9d4e6e@suse.com> <9eb39857-2e33-4a6b-1825-f9dc537a6515@xen.org> <423c0369-9c90-dbfe-2f90-d49a2ce5b283@suse.com> <4cd108f9-3ad0-2262-fa7c-d2247660c635@xen.org> <33f4c492-6400-6386-8dbe-b6b098e97fec@suse.com> From: Jan Beulich Message-ID: <1494fe06-b353-00a5-17a6-c11cee269519@suse.com> Date: Wed, 29 Apr 2020 17:23:01 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Juergen Gross , Stefano Stabellini , Wei Liu , Andrew Cooper , Julien Grall , Ian Jackson , George Dunlap , xen-devel@lists.xenproject.org Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 29.04.2020 17:06, Julien Grall wrote: > > > On 29/04/2020 15:56, Jan Beulich wrote: >> On 29.04.2020 16:14, Julien Grall wrote: >>> Hi Jan, >>> >>> On 29/04/2020 15:05, Jan Beulich wrote: >>>> On 29.04.2020 16:01, Julien Grall wrote: >>>>> Hi, >>>>> >>>>> On 22/04/2020 10:20, Jan Beulich wrote: >>>>>>> Even if it was possible to use the sub-structs defined in the header >>>>>>> that way, keep in mind that we also wrote: >>>>>>> >>>>>>>            /* dummy member to force sizeof(struct xen_pvcalls_request) >>>>>>>             * to match across archs */ >>>>>>>            struct xen_pvcalls_dummy { >>>>>>>                uint8_t dummy[56]; >>>>>>>            } dummy; >>>>>> >>>>>> This has nothing to do with how a consumer may use the structs. >>>>>> >>>>>>> And the spec also clarifies that the size of each specific request is >>>>>>> always 56 bytes. >>>>>> >>>>>> Sure, and I didn't mean to imply that a consumer would be allowed >>>>>> to break this requirement. Still something like this >>>>>> >>>>>> int pvcall_new_socket(struct xen_pvcalls_socket *s) { >>>>>>        struct xen_pvcalls_request req = { >>>>>>            .req_id = REQ_ID, >>>>>>            .cmd = PVCALLS_SOCKET, >>>>>>            .u.socket = *s, >>>>>>        }; >>>>>> >>>>>>        return pvcall(&req); >>>>>> } >>>>>> >>>>>> may break. >>>>> >>>>> I think I understand your concern now. So yes I agree this would break 32-bit consumer. >>>>> >>>>> As the padding is at the end of the structure, I think a 32-bit frontend and 64-bit backend (or vice-versa) should currently work without any trouble. The problem would come later if we decide to extend a command. >>>> >>>> Can commands be extended at all, i.e. don't extensions require new >>>> commands? The issue I've described has nothing to do with future >>>> extending of any of the affected structures. >>> >>> I think my point wasn't conveyed correctly. The implicit padding is at >>> the end of the structure for all the consumers but 32-bit x86. So >>> without any modification, I think 32-bit frontend can still communicate >>> with 64-bit backend (or vice-versa). >> >> There's no issue communicating afaics, as for communication >> you wouldn't use the sub-structures, but the single container >> one. The problem is, as described, with possible uses internal >> to one side of the communication. > > I am sorry but I can't figure out how this is an issue. The > problem you described would only happen if you are calling a > 64-bit library from a 32-bit software. Why? The example given doesn't require such. > Is it even possible? In principle yes, I think. I don't think OSes like Linux allow this, though. Jan